Highly efficient gallium nitride based light emitting diodes via surface roughening

ABSTRACT

A gallium nitride (GaN) based light emitting diode (LED), wherein light is extracted through a nitrogen face (N-face) of the LED and a surface of the N-face is roughened into one or more hexagonal shaped cones. The roughened surface reduces light reflections occurring repeatedly inside the LED, and thus extracts more light out of the LED. The surface of the N-face is roughened by an anisotropic etching, which may comprise a dry etching or a photo-enhanced chemical (PEC) etching.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. Section 120 of thefollowing co-pending and commonly-assigned application:

U.S. Utility application Ser. No. 16/561,366, filed on Sep. 5, 2019, byTetsuo Fujii, Yan Gao, Evelyn. L. Hu, and Shuji Nakamura, entitled“HIGHLY EFFICIENT GALLIUM NITRIDE BASED LIGHT EMITTING DIODES VIASURFACE ROUGHENING”, now U.S. Pat. No. 10,985,293, issued Apr. 20, 2021,which application is a continuation under 35 U.S.C Section 120 of thefollowing co-pending and commonly-assigned application:

U.S. Utility application Ser. No. 14/281,535, filed on May 19, 2014, byTetsuo Fujii, Yan Gao, Evelyn. L. Hu, and Shuji Nakamura, entitled“HIGHLY EFFICIENT GALLIUM NITRIDE BASED LIGHT EMITTING DIODES VIASURFACE ROUGHENING”, now U.S. Pat. No. 10,446,714, issued Oct. 15, 2019,which application is a continuation under 35 U.S.C. Section 120 of thefollowing co-pending and commonly-assigned application:

U.S. Utility application Ser. No. 12/576,122, filed on Oct. 8, 2009, byTetsuo Fujii, Yan Gao, Evelyn. L. Hu, and Shuji Nakamura, entitled“HIGHLY EFFICIENT GALLIUM NITRIDE BASED LIGHT EMITTING DIODES VIASURFACE ROUGHENING”, now U.S. Pat. No. 8,766,296, issued Jul. 1, 2014,which application is a continuation under 35 U.S.C. Section 120 of thefollowing co-pending and commonly-assigned application:

U.S. Utility application Ser. No. 10/581,940, filed on Jun. 7, 2006, byTetsuo Fujii, Yan Gao, Evelyn. L. Hu, and Shuji Nakamura, entitled“HIGHLY EFFICIENT GALLIUM NITRIDE BASED LIGHT EMITTING DIODES VIASURFACE ROUGHENING”, now U.S. Pat. No. 7,704,763, issued Apr. 27, 2010,which application claims the benefit under 35 U.S.C. Section 365(c) ofthe following co-pending and commonly-assigned application:

PCT Application Serial No. US2003/039211, filed on Dec. 9, 2003, byTetsuo Fujii, Yan Gao, Evelyn L. Hu, and Shuji Nakamura, entitled“HIGHLY EFFICIENT GALLIUM NITRIDE BASED LIGHT EMITTING DIODES VIASURFACE ROUGHENING”;

all of which applications are incorporated by reference herein.

1. FIELD OF THE INVENTION

The invention is related to light emitting diodes, and moreparticularly, to highly efficient gallium nitride based light emittingdiodes via surface roughening.

2. DESCRIPTION OF THE RELATED ART

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numbers.A list of these different publications ordered according to thesereference numbers can be found below in the section entitled“References.” Each of these publications is incorporated by referenceherein.)

Gallium nitride (GaN) based wide band gap semiconductor light emittingdiodes (LEDs) have been available for about 10 years. The progress ofLED development has brought about great changes in LED technology, withthe realization of full-color LED displays, LED traffic signals, whiteLEDs and so on

Recently, high-efficiency white LEDs have gained much interest aspossible replacements for fluorescent lamps. Specifically, theefficiency of white LEDs (74 lm/W) [1] is approaching that of ordinaryfluorescent lamps (75 lm/W). Nonetheless, more improvement in efficiencyis desirable.

There are two principle approaches for improving LED efficiency. Thefirst approach is increasing the internal quantum efficiency (η_(i)),which is determined by crystal quality and epitaxial layer structure,while the second approach is increasing the light extraction efficiency(η_(extraction)).

Increasing the internal quantum efficiency cannot readily be done. Atypical η_(i) value for blue LEDs is more than 70% [2] and anultraviolet (UV) LED grown on a low-dislocation GaN substrate hasrecently exhibited an η_(i) of about 80% [3]. There is little room forimprovement of these values.

On the other hand, there is plenty of room for improving the lightextraction efficiency. A number of issues may be addressed ineliminating the internal loss of light, including: high reflectivemirror, low reflection surface such as roughened surface, highly thermaldispersion structure, etc.

For example, considering the refractive indices of GaN (n≈2.5) [4] andair, the critical angle for the light escape cone is about 23°. Assumingthat light emitted from sidewalls and the backside is neglected, it isexpected that approximately only 4% of the internal light can beextracted. The light outside the escape cone is reflected into thesubstrate and is reflected repeatedly or absorbed by active layers orelectrodes, unless it escapes through the sidewalls.

The LED structure affects how much light is emitted. The impact of LEDstructure on light extraction efficiency is best described by example.The following examples describe several types of LED structures.

FIG. 1 is a schematic cross-section of a conventional LED structure,which includes a p-type pad electrode 10, semitransparent electrode 12,p-type layer 14, active region 16, n-type layer 18, n-type electrode 20,and substrate 22. Because GaN is usually grown on an insulatorsubstrate, such as sapphire, p-type and n-type electrodes 10, 20 need tobe fabricated on the same plane and the resulting device structure ofthe electrodes 10, 20 imposes a lateral current flow. Due to the highresistivity of p-type GaN, a thin metal film is employed as asemitransparent electrode 12 for current spreading on the p-type GaN. Itis desirable that the transparency of the semitransparent electrode 12should be 100%; however, its' value for the thin metal electrodes usedin GaN based LEDs is 70% at most. Moreover, the pad electrode 10 shouldbe formed for wire bonding, which obscures the light emitted from theinside of the LED; consequently, the extraction efficiency is expectedto be quite low.

FIG. 2 is a schematic cross-section of a flip-chip type LED structure,which includes a transparent sapphire substrate 24, n-type layer 26,n-type electrode 28, active region 30, p-type layer 32, p-type electrode34, solder 36, and host submount 38. In order to improve the externalefficiency, light can be extracted though the transparent sapphiresubstrate 24 of the flip-chip type LED structure. This method has anadvantage over conventional LEDs with respect to a reduction of thelight absorption by the thin metal film and the pad electrode. However,most of the light emitted from the active region would be reflected atthe interface between the substrate 24 and n-type layer 26, and theinterface between the substrate 24 and the air.

A method that allows for GaN film detachment from a sapphire substrateis called “laser lift off” (LLO) technique. By applying this method toflip-chip type GaN based LEDs, sapphire substrate-free GaN LEDs can berealized. Assuming that the resulting GaN surface is worked into anon-planar orientation, a significant improvement of the extractionefficiency is expected.

Another approach to increasing extraction efficiency is to roughen theLED's surface [5], which discourages internal light reflection andscatters the light upward. However, surface roughened LEDs have beenmentioned only in the context of the gallium phosphide (GaP) family ofmaterials, because GaN is very durable material and an ordinary wetetching method does not have much effect. Thus, although the idea ofroughening the semiconductor surface for the sake of scattering lightwas first considered in the 1970's, it has been believed to be difficultand costly for this kind of LED structure to be produced.

However, as noted above, typical GaN-based LEDs are comprised of a thinp-GaN/active layer/n-GaN film on a sapphire or silicon carbide (SiC)substrate. Although producing a roughened surface requires a certain GaNlayer thickness [6], growing thick p-GaN is not desirable due to therelatively high resistivity of p-GaN, which demands a semi-transparencycontact on the p-GaN surface if the light is extracted through thep-GaN, and some treatments such as dry etching [7] for rougheningsurfaces might cause electrical deterioration. Growing a p-side downstructure by metalorganic chemical vapor deposition (MOCVD) is alsoundesirable, because of the magnesium (Mg) memory effect [8], whichdeteriorates the active layer.

Recently, a laser lift off (LLO) method has been used to detach asapphire substrate from a GaN film grown on the substrate [9-11].Further, LLO has been used to fabricate GaN-based LEDs [12, 13].However, there was no reference to the effect of this technique onsurface morphology or extraction efficiency.

On the other hand, in the present invention, utilizing flip-chiptechnology [14] and the LLO method, a substrate-free nitrogen (N)side-up GaN-based LED structure can be made. Thereafter, an anisotropicetching process can be used to roughen the surface of the N-side-upGaN-based LED. This results in a hexagonal “cone-like” surface, which isbeneficial for light extraction. Extraction efficiency of an optimallyroughened surface LED shows an increase by more than 100% compared to anLED before roughening.

Note that, for some time, GaN has been believed to be difficult toanisotropically etch. This is true because GaN is a chemically stablematerial compared with other semiconductor materials. The use of dryetching to make a textured surface is possible, but requires extraprocessing, such as photolithography, and it is impossible to make afine cone-like surface on the GaN.

When photo-enhanced chemical (PEC) etching is used on gallium face(Ga-face) GaN, small pits are formed on the surface. This is in contrastto PEC etching of nitrogen face (N-face) GaN, which results in distinctcone-like features. Although there are a few reports dealing withGaN-based LEDs fabricated using the LLO technique, the present inventionfabricates cone-like structures on the N-face GaN surface of the GaNbased LED using an anisotropic etching method.

SUMMARY OF THE INVENTION

The present invention describes a gallium nitride (GaN) based lightemitting diode (LED), wherein light is extracted through a nitrogen face(N-face) of the LED and a surface of the N-face is roughened into one ormore hexagonal shaped cones. The roughened surface reduces lightreflections occurring repeatedly inside the LED, and thus extracts morelight out of the LED.

The surface of the N-face is roughened by an anisotropic etching. Theanisotropic etching may comprise a dry etching or a photo-enhancedchemical (PEC) etching.

In one embodiment, the N-face GaN is prepared by a laser lift off (LLO)technique. In another embodiment, the LED is grown on a c-plane GaNwafer, a p-type layer's surface is a gallium face (Ga-face), and then-type layer's surface is a nitrogen face (N-face).

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a schematic cross-section of a conventional LED structure;

FIG. 2 is a schematic cross-section of a flip-chip type LED structure;

FIG. 3 is a schematic of a surface roughened LED;

FIG. 4 is a flowchart that illustrates the processing steps used in thepreferred embodiment of the present invention;

FIGS. 5A, 5B, 5C, 5D, 5E and 5F further illustrate the fabrication stepsfor the LEDs with surface roughening;

FIG. 6A shows an LED with a current-blocking layer, while FIG. 6B showsan LED with a current-confining frame;

FIGS. 7A and 7B are plan-view micrographs of an LLO-LED with across-shaped n-electrode;

FIGS. 8A and 8B are scanning electron micrograph (SEM) images of theN-face of GaN after PEC etching for different etching times;

FIGS. 9A and 9B show an electroluminescence (EL) spectra from aflat-surface LED and a roughened-surface LED, respectively; and

FIG. 10 is a graph of upward EL output power vs. DC injection current(L-I) characteristics for the LEDs with different etching times at roomtemperature.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

The present invention provides a means of increasing the extractionefficiency by roughening the surface of GaN based LEDs. Specifically,applying an anisotropic PEC etching method to an N-face c-plane GaNsurface results in the fabrication of cone-shaped surface features. Thisroughened surface reduces light reflections occurring repeatedly insidethe LED, and thus extracts more light out of the LED. Moreover, themethod of the present invention is simple, repeatable and should notdamage the material, in contrast to other methods of surface rougheningthat may compromise the material's quality, all of which renders thepresent invention more suitable for manufacturing LEDs.

LED Structure

FIG. 3 is a schematic of a surface roughened LED, which includes ann-type electrode 40, n-type layer 42, active region 44, p-type layer 46and p-type electrode 48 which has been flip-chip bonded via a solderlayer 50 to a silicon (Si) submount 52 that includes an n-type electrode54. The n-type layer 42, active region 44 and p-type layer 46 arecomprised of a (B, Al, Ga, In)N alloy. A dry or PEC etching method isused to roughen the surface of the n-type layer 42. Appropriateconditions, such as plasma chemistries and plasma power for dry etching,and electrolytes and lamp power for PEC etching, need to be set so thata desirable surface can be obtained. It is important that this GaN basedLED should be grown along its c-axis and this n-type GaN surface shouldbe N-face because anisotropic etching can be observed on N-face GaN muchmore readily than Ga-face GaN.

Note that c-plane GaN is the structure where the plane that containsonly Ga atoms and the plane that contains only N atoms are piled orstacked up alternately. If one surface is Ga-face, then the opposingsurface is N-face. Due to the fact that Ga-face c-plane GaN is generallypreferred from the point of view of crystal growth and deviceperformance, N-face GaN needs to be prepared by the LLO technique, oralternatively, the LED structure could be grown on the c-plane bulk GaNwafer.

The light emitted from the active region 44 toward the roughened n-typeGaN surface 42 is scattered by the surface, which does not reflect thelight back to the active region. It is desired that the p-type electrode48 have a property of high reflection to decrease light absorption andto increase light reflection toward the n-type GaN surface 42. Inexperimental results, it has been determined that the present inventionincreases the upward light output power for the LED with a roughenedsurface two or three times as compared with an LED with a flat surface.

Processing Steps

FIG. 4 is a flowchart that illustrates the processing steps used in thepreferred embodiment of the present invention.

Block 56 represents the step of growing Ga-face epitaxial layers on ac-plane sapphire substrate by MOCVD, thereby creating a sample.

Block 58 represents the step of annealing the sample for p-typeactivation, after the MOCVD.

Block 60 represents the step of performing a p-type metallizationprocess on the sample, including, but not limited to, silver (Ag) oraluminum (Al), to create a highly reflective p-GaN contact.

Block 62 represents the step of depositing thick gold (Au) layers on thesample, followed by tin (Sn) layers as a solder metal by Sn evaporationin a thermal evaporator.

Block 64 represents the step of flipping the sample and bonding it to anAu-coated Si substrate/submount at a temperature above 280° C., whereinan Au/Sn alloy forms that contributions to the adhesion of the sample tothe Si substrate.

Block 66 represents the step of performing an LLO process by irradiatingthe transparent sapphire substrate of the sample using a kryptonfluoride (KrF) eximer laser light (248 nm) through the backside of thesapphire substrate, resulting in local decomposition of the GaN at theGaN/sapphire substrate interface. Specifically, by rastering the KrFeximer laser spot over the sample, the GaN-based LED membrane istransferred to the Si substrate/submount.

Block 68 represents the step of debonding the sapphire substrate fromthe sample, after rastering the KrF laser over the sample.

Block 70 represents the step of removing any residual Ga droplets on thedetached GaN surface of the sample using an hydrochloride (HCl)solution.

Block 72 represents the step of thinning the transferred GaN until theSi-doped N-face GaN is exposed on the sample.

Block 74 represents the step of depositing atitanium/aluminum/titanium/gold (Ti/Al/Ti/Au) electrode as an n-typecontact or electrode on the exposed N-face GaN of the sample.

Block 76 represents the step of PEC etching by immersing the sample inan electrolyte solution of potassium hydroxide (KOH) and irradiating theN-face GaN surface using a xenon/mercury (Xe/Hg) lamp, in such a waythat the top surface is roughened. The details of PEC etching aredescribed in [15].

Block 78 represents the step of separating each device on the Sisubstrate of the sample using a dry etching, dicing or cleaving method.

FIGS. 5A, 5B, 5C, 5D, 5E and 5F further illustrate the fabrication stepsfor the LEDs with surface roughening, wherein the LED structure includesa p-type electrode 80, GaN-based LED membrane 82, sapphire substrate 84,solder metal 86, submount (carrier) 88 and n-type electrode 90.Specifically, FIG. 5A shows the results after the p-type electrode 80deposition, FIG. 5B shows the results after the LED is bonded onto thehost submount 88, FIG. 5C shows the results after the sapphire substrate84 removal by LLO, FIG. 5D shows the results after n-type electrode 90deposition, FIG. 5E shows the results after the roughening of the GaNsurface 82, and FIG. 5F shows the results after device isolation.

Possible Modifications

Although a basic structure has been described above, a number ofmodifications and variations are possible.

FIG. 6A shows an LED with a current-blocking layer, while FIG. 6B showsan LED with a current-confining frame, wherein the LEDs include ann-type electrode 92, n-type layer 94, active layer 96, p-type layer 98,p-type electrode 100, current-blocking layer 102, and current confiningframe 104.

In FIG. 6A, the LED has a current-blocking layer 102 aligned under then-type electrode 92. This current-blocking layer 102 keeps the currentfrom concentrating below the n-type electrode 92 so that absorption oflight emission under the electrode 92 can be avoided and the extractionefficiency can be increased. It is suitable that an insulator such asSift is located on the p-GaN layer 98 because the current spreadinghardly occurs in the resistive p-GaN layer 98.

In FIG. 6B, the LED has a current-confining frame 104 made of aninsulator. If a dry-etching or a dicing method is used to separate thedevices, the sidewalls of the devices might conduct a leakage current,if the surfaces are damaged. Such leakage current decreases both theefficiency and lifetime of the LED. The current-confinement frame 104contributes to the restraint of leakage current through the sidewalls ofthe LED and does not significantly decrease the emitting area, if thewidth of the frame is chosen appropriately.

Although an Si substrate has been described as a host submount in theLLO process, alternative substrate materials may be used to practicethis invention. Although Si is cheaper and has a higher thermalconductivity than sapphire, other substrates, such as SiC, diamond, AlN,or various metals such as CuW, may be fit for use from the point of viewof thermal conductivity.

At present, GaN devices can be also grown directly on SiC and Sisubstrate. If a GaN-based LED is grown on SiC or Si, conventional dryetching or wet etching can remove the substrate. By utilizing a bulk GaNsubstrate, the LLO process can be eliminated.

Sample size is also an important point for LED fabrication. Nowadays,LEDs with a large size are attracting attention to meet demand forhigh-power LEDs. Even though the resistivity of the n-type GaN is lowerthan that of p-GaN, the size affects the n-type electrode geometry forthe purpose of current spreading.

Experimental Results

In experiments performed by the inventors, Ga-face epitaxial layers weregrown on a c-plane sapphire substrate by MOCVD. The structure wascomprised of 4 μm-thick undoped and Si-doped GaN layers, a 5-periodGaN/InGaN multi-quantum-well (MQW), a 20 nm-thick Mg-dopedAl_(0.2)Ga_(0.8)N layer, and 0.3 μm-thick Mg-doped GaN. After MOCVD, thesample was annealed for p-type activation and then a p-typemetallization process was performed. An Ag-based electrode was adoptedas a highly reflective p-GaN contact. Thick Au was deposited on thesample followed by Sn evaporation in a thermal evaporator. The wafer wasflipped and bonded to an Au-coated Si submount at a temperature of 280°C., resulting in an alloy of Au and Sn, which contributes to firmadhesion of the wafer to the submount. A KrF laser (248 nm) was used forthe LLO process, in which the laser was shone through the transparentsapphire substrate, causing local decomposition of GaN at the boundarybetween GaN and sapphire. After rastering the KrF laser over the sample,the sapphire substrate was debonded. The remaining Ga droplets on thetransferred GaN surface were removed by an HCl solution. Next, thetransferred GaN was thinned until the Si-doped GaN was exposed. Ann-contact was formed on the exposed N-face n-GaN and each device wasdivided from its neighbors by reactive ion etching (RIE). Finally, inorder to roughen the top of surface, PEC etching was used. A KOHsolution and Xe/Hg lamp were used as electrolyte and light source,respectively. The output power of the LED was measured with anSi-detector set at a height of 7 mm over the LED chips.

FIGS. 7A and 7B are plan-view micrographs of an LLO-LED with across-shaped n-electrode, wherein the LED is bonded on an Si substrate.FIG. 7A shows the surface before roughening and FIG. 7B shows thesurface after roughening. Because the n-electrode blocks UV light duringPEC etching, the GaN beneath it is not etched and the electrode remainson the GaN after roughening. A transparent electrode such as indium tinoxide (ITO) can be employed as a current spreading electrode.

FIGS. 8A and 8B are scanning electron micrograph (SEM) images of theN-face of GaN after PEC etching for different etching times. Notice thatthe PEC-etched N-face GaN surfaces include a plurality of hexagonalshaped cones, which are distinct from the PEC-etched GaN surfacesreported by Youtsey, et al. [16]. This difference is considered to bedue to the surface polarity of GaN. In comparing the 2 minute (min)etched surface of FIG. 8A and the 10 min etched surface of FIG. 8B, thesize of the features increases and the facets of the hexagonal conesbecome more defined.

The cone-shaped surface appears very effective for light extraction fromthe LED. Moreover, experimental results suggest that a cone shape canextract more light. For example, the wavelength of a blue LED in a GaNcrystal is about 200 nm. If the size of the cone shape is much smallerthan that value, then the light might not be affected by the roughness.On the other hand, if the size of the cone shape is close to that value,the light might be scattered or diffracted.

In experimental results, it has been determined that the roughenedsurface is comprised of many hexagonal shaped cones that have an angleequal to or smaller than:2 sin⁻¹(n_(air)/n_(s))≈47.2°for GaN, where n_(air) is a refractive index of air and n_(s) is arefractive index of GaN. Similarly, it has been determined that theroughened surface is comprised of many hexagonal shaped cones that havean angle equal to or smaller than:2 sin⁻¹(n_(enc)/n_(s))for epoxy, where n_(enc) is a refractive index of epoxy and n_(s) is arefractive index of GaN.

It is possible that the surface may not have to be cone shaped, and agrating structure and photonic crystal should be considered. These mightbe better structures for light extraction. However, the fabrication ofphotonic crystals requires precise design and processing, which is morecostly than fabricating a cone-shaped surface roughness.

The “mirror-like” surface before PEC etching becomes discolored as theetching time increases. If a highly reflective metal is deposited on theother side of GaN film, the surface appears white; otherwise, it isdarker. This is believed to be due to the light-reflection restraint atthe air/GaN boundary, and if there is a highly reflective metal on thebackside of the GaN, the light passing into GaN comes out again,scattering at the roughened surface.

Electroluminescence (EL) spectra from a flat-surface LED and aroughened-surface LED are shown in FIGS. 9A and 9B, respectively. Themeasurement was performed at a forward current density of 25 A/cm² DC atroom temperature (RT). The spectrum of the flat-surface LED hadmulti-peaked emission, as shown in FIG. 9A, suggesting that the lightemitted from the active region was interfered in the vertical GaN cavitysandwiched between mirrors made of GaN/metal and GaN/air. In contrast,as shown in FIG. 9B, no longitudinal mode was observed on the roughenedsurface LED. This means that the roughened GaN/air interface scatteredthe light, resulting in suppression of the resonance.

FIG. 10 is a graph of upward EL output power vs. DC injection current(L-I) characteristics for the LEDs with different etching times at roomtemperature. These data were obtained from the same device before andafter PEC etching, so that any factor causing this difference except thesurface morphology could be neglected. Any L-I curves showed linearcharacteristics up to 50 mA. Because of the relatively higher thermalconductivity of Si than that of sapphire, these devices are advantageousfor high power operation. The output power at a given current increasedwith increasing PEC etching time. As compared with the output power fora flat-surface LED and the 10 min etched surface LED, this rougheningtreatment resulted in an increase of output power by a factor of 2.3.From other measurements on different devices, the power also showed atwo to three-fold increase after the roughening process. Because aflat-surface LED tends to emit more light from the sidewalls of the LEDchip than a roughened surface LED due to the lateral propagation oflight, the difference of output power would be less if the total powerwere measured in an integrating sphere. Nevertheless this enhancement ofextraction efficiency by anisotropic etching technique shows significantimprovement.

In conclusion, an anisotropic etching method has been applied to aGaN-based LED for the purpose of increasing extraction efficiency. LEDoutput test results have indicated that, presumably due to the decreasein light propagation in the GaN film, there is a relationship between aroughened appearance and extraction efficiency. Although totalintegrated optical power has not been measured, the largest increase inextraction efficiency was more than 100%. It is notable that thetechnique described herein is simple and does not require complicatedprocesses, which indicates that it will be suitable for manufacturingGaN based-LEDs with surface roughening.

REFERENCES

The following references are incorporated by reference herein:

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CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The following describes some alternative embodimentsfor accomplishing the present invention.

A number of different growth methods other than MOCVD could be used inthe present invention.

In addition, substrates other than sapphire or silicon carbide could beemployed.

Also, different LED structures may be created. For example, resonantcavity LEDs (RCLEDs) or micro cavity LEDs (MCLEDs) could be created aswell.

The foregoing description of one or more embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

What is claimed is:
 1. A device, comprising: a gallium nitride (GaN)based light emitting diode (LED) including at least an n-type layer, anemitting layer, and a p-type layer; wherein light from the emittinglayer is extracted through a nitrogen face (N-face) surface of at leastone of the layers of the LED, the N-face surface is roughened, and theroughened N-face surface extracts more light out of the LED byscattering or diffracting the light.
 2. The device of claim 1, whereinthe LED is grown on a sapphire substrate, the p-type layer's surface isa gallium face (Ga-face), and the n-type layer's surface is a nitrogenface (N-face).
 3. The device of claim 2, wherein the N-face surface isprepared for roughening by a laser lift off (LLO) of the sapphiresubstrate.
 4. The device of claim 1, wherein the N-face surface isroughened by an anisotropic etching.
 5. The device of claim 4, whereinthe anisotropic etching comprise a dry etching or a photo-enhancedchemical (PEC) etching.
 6. The device of claim 1, wherein the N-facesurface is roughened into one or more structures.
 7. The device of claim6, wherein the structures have a size close to a wavelength of the lightwithin the LED.
 8. The device of claim 7, wherein the structures have asize not smaller than a wavelength of the light within the LED.
 9. Thedevice of claim 6, wherein the structures comprise cone shapes.
 10. Thedevice of claim 6, wherein the structures comprise hexagonal shapedcones that have an angle equal to or smaller than:2 sin⁻¹(n_(air)/n_(s)) where n_(air) is a refractive index of air andn_(s) is a refractive index of the N-face surface.
 11. The device ofclaim 1, wherein the extraction efficiency of the light out of theroughened N-face surface is increased by more than 100% as compared toan N-face surface that is not roughened.
 12. A method, comprising:fabricating a gallium nitride (GaN) based light emitting diode (LED)including at least an n-type layer, an emitting layer, and a p-typelayer; wherein light from the emitting layer is extracted through anitrogen face (N-face) surface of at least one of the layers of the LED,the N-face surface is roughened, and the roughened N-face surfaceextracts more light out of the LED by scattering or diffracting thelight.
 13. The method of claim 12, wherein the LED is grown on a c-planeGaN or sapphire substrate, the p-type layer's surface is a gallium face(Ga-face), and the n-type layer's surface is the N-face surface.
 14. Themethod of claim 13, wherein the N-face surface is prepared forroughening by a laser lift off (LLO) of the substrate.
 15. The method ofclaim 12, wherein the N-face surface is roughened by an anisotropicetching.
 16. The method of claim 15, wherein the anisotropic etchingcomprise a dry etching or a photo-enhanced chemical (PEC) etching. 17.The method of claim 12, wherein the N-face surface is roughened into oneor more structures.
 18. The method of claim 17, wherein the structureshave a size close to a wavelength of the light within the LED.
 19. Themethod of claim 17, wherein the structures have a size not smaller thana wavelength of the light within the LED.
 20. The device of claim 17,wherein the structures comprise hexagonal shaped cones that have anangle equal to or smaller than:2 sin⁻¹(n_(air)/n_(s)) where n_(air) is a refractive index of air andn_(s) is a refractive index of the N-face surface.